Hydrocarbons, such as oil, may be recovered from hydrocarbon containing formations (or reservoirs) by penetrating the formation with one or more wells, which may allow the hydrocarbons to flow to the surface. A hydrocarbon containing formation may have a natural energy source (for example gas, water) to aid in mobilising hydrocarbons to the surface of the wells. For example, water or gas may be present in the formation at sufficient levels to exert pressure on the hydrocarbons to mobilise them to the surface of the production wells. However, reservoir conditions (for example permeability, hydrocarbon concentration, porosity, temperature, pressure) can significantly impact the economic viability of hydrocarbon production from any particular hydrocarbon containing formation. Furthermore, any natural energy sources that exist may become depleted over time, often long before the majority of hydrocarbons have been extracted from the reservoir. Therefore, supplemental recovery processes may be required and used to continue the recovery of hydrocarbons from the hydrocarbon containing formation. Examples of known supplemental processes include waterflooding, polymer flooding, gas flooding, alkali flooding, thermal processes, solution flooding or combinations thereof.
In recent years there has been increased activity in developing new and improved methods of chemical Enhanced Oil Recovery (cEOR) for maximising the yield of hydrocarbons from a subterranean reservoir. In surfactant cEOR the mobilisation of residual oil saturation is achieved through surfactants which generate a sufficiently (ultra) low crude oil/water interfacial tension (IFT) to give a capillary number large enough to overcome capillary forces and allow the oil to flow (Chatzis & Morrows, “Correlation of capillary number relationship for sandstone”, SPE Journal, volume 29, pages 555-562, 1989). However, different reservoirs can have very different characteristics (for example crude oil type, temperature, water composition—salinity, hardness etc.), and therefore, it is desirable that the structures and properties of the added surfactant(s) be matched to the particular conditions of a reservoir to achieve the required low IFT. In addition, other important criteria must be fulfilled, such as low rock retention or adsorption, compatibility with polymer, thermal and hydrolytic stability and acceptable cost (including ease of commercial scale manufacture).
Compositions and methods for cEOR utilising an internal olefin sulfonate (IOS) as surfactant are described in U.S. Pat. No. 4,597,879, U.S. Pat. No. 4,979,564, U.S. Pat. No. 5,068,043 and “Field Test of Cosurfactant-enhanced Alkaline Flooding”, Falls et al., Society of Petroleum Engineers Reservoir Engineering, 1994.
Savings can be accomplished if a hydrocarbon recovery composition, containing an IOS as a surfactant, is in such a form that it can easily be transported to the hydrocarbon recovery location and then easily stored at that location, and further that at the hydrocarbon recovery location limited equipment is needed to produce the fluid that is provided to the hydrocarbon containing formation.
Surfactants for enhanced hydrocarbon recovery are normally provided to the hydrocarbon containing formation by admixing it with water and/or brine which may originate from the formation from which hydrocarbons are to be recovered, thereby forming a fluid that can be injected into the hydrocarbon containing formation. The surfactant amount in such injectable water containing fluid is generally in the range of from 0.1 to 1 wt. %. See for example “Favorable Attributes of Alkaline-Surfactant-Polymer Flooding”, Liu et al., SPE Journal, March 2008, pages 5-16.
Surfactants for enhanced hydrocarbon recovery are normally synthesised at a location which is far remote from the location where hydrocarbons are to be recovered from a hydrocarbon containing formation. This means that the surfactants have to be transported to that hydrocarbon recovery location. Such transport involves high costs. It is desirable to find a way which substantially reduces these costs other than by synthesising the surfactants at the hydrocarbon recovery location itself. The latter option is not cost efficient because in such case there would still be transport involved for surfactants to be provided to other hydrocarbon recovery locations.
Normally, surfactants for enhanced hydrocarbon recovery are transported to a hydrocarbon recovery location and stored at that location in the form of an aqueous solution containing for example 30 to 35 wt. % of the surfactant. At the hydrocarbon recovery location, such solution would then be further diluted to a 0.1-1 wt. % solution as referred to above, before it is injected into a hydrocarbon containing formation. In practice, it is preferred that the surfactant containing aqueous mixtures are not too viscous because then they would be difficult to handle in said transport, storage and dilution. Therefore, in practice, it is not preferred to transport and store more concentrated (for example 60-80 wt. % instead of said 30-35 wt. %) surfactant containing aqueous mixtures because such mixtures are generally highly viscous and therefore difficult to handle in said transport, storage and dilution.
It is important that surfactants for enhanced hydrocarbon recovery are injected into a hydrocarbon containing formation as part of a single-phase solution, because formation of precipitate, liquid crystal or a second liquid phase can lead to non-uniform distribution of injected material and non-uniform transport owing to phase trapping or different mobilities of coexisting phases. This means that a physically stable, that is to say a homogeneous, solid-free surfactant containing solution should be obtained and that such surfactant containing solution should remain homogeneous and solid-free during said transport, storage and dilution.
Therefore, it is desired to find a way which substantially reduces costs for transporting and storing IOS surfactant for enhanced hydrocarbon recovery, while at the same time such IOS surfactant can still be provided to a hydrocarbon containing formation as part of a single-phase solution and still have an acceptable cEOR performance, for example in terms of reducing the IFT, as described above.